Who? Everyone in the video is running by 6 seconds in, and it reasonably takes 2-3 seconds to be clear that this isn't just a normal geyser eruption.
Some of the runners are shambling or in a very light jog, but that looks like it's down to form or fitness rather than being blasé. I don't see anyone "strolling".
The woman in the white shirt, tan shorts only started running because the people behind her started running. She was strolling away from the explosion. Same with green shorts in the foreground. We don't actually see him run, just walking away.
These people have no sense of urgency around danger, or they cannot recognize danger.
Some of the runners are shambling or in a very light jog, but that looks like it's down to form or fitness rather than being blasé. I don't see anyone "strolling".